None of my points have anything to do with cost, I'm (realistically) assuming a campaign with huge amounts of cash. The problem, as with most conspiracy theories, is the number of people that need to cooperate and stay silent, and the number of independent safeguards that already exist. Spending more money actually hurts you, because it makes it harder to hide where that money is going, especially with all of the campaign finance regulations.

I can't think of a low-risk way to literally hack an election in the US at the moment. You would need a long-term plan to change election laws, homogenize voting machines and methods across the country, and then hack some centralized database.

There is no way to get around the polling issue in a country with an even semi-free press, so you still need a pretty strong candidate that you just boost a little.

Anybody in the real world who needed to rig an election would use indirect methods. Spying, spreading misinformation, ignoring campaign finance laws, gerrymandering, etc. All things that have happened and will continue to happen.